1. Technical Field
The present disclosure is directed to thin film resistors, and more particularly, to a thin film resistor having a dielectric cap, where the thin film resistor electrically connects conductive layers of adjacent interconnects.
2. Description of the Related Art
Precision resistors provide stable resistances for integrated circuits used in various precision electronic devices, such as pacemakers, printers, and testing or measuring instruments. Each electronic device utilizes specific resistance values and operates in different conditions. Manufacturers tailor precise resistance values for each electronic device by controlling the size of the resistor and by using materials having low temperature and voltage coefficients. However, the performance of these precision resistors is often impacted by variations in operating conditions, like temperature and voltage. Manufacturers strive to achieve tight tolerances with respect to resistance and size to better attain precise, stable resistances.
Conventional precision resistors include diffused resistors and laser trimmed polysilicon resistors. Diffused resistors have a dopant introduced into a polysilicon resistor layer in the substrate, forming a doped active region, such as a P-well or P-body in the substrate. High-ohmic polysilicon resistors have temperature coefficients of resistance in the range of 1,000 and 3,000 parts per million per degree Celsius with resistances in the range of 1 k and 10 k ohms/square. In addition, the resistance of a doped polysilicon layer changes with temperature because the carriers are activated, which can cause performance drifts that follow the operating temperature.
A length and width of the doped resistor layer, a depth of diffusion, and a resistivity of the dopant control the specific resistance achieved. Junction isolation techniques isolate the diffused resistor from other elements in the substrate. These isolation techniques, which take up precious space on the substrate, minimize the adverse impacts of space charge effects of p-n junctions that can alter the resistance as the operating voltage and frequency change. To compensate for these changes in resistance, manufacturers often include additional circuitry adjacent the resistor, thereby using more substrate area surrounding the resistor.
Laser trimming removes or cuts away portions of a polysilicon resistor layer to increase the resistance. More particularly, the laser alters the shape of the resistor to achieve a desired resistance value. As with diffused resistors, laser trimmed resistors use large areas of the substrate in order to achieve precise resistor values. The large area dimensions also allow these resistors to dissipate heat to the substrate. The size requirements of these resistors impact the density of devices in the integrated circuit. As a result of the continued miniaturization of integrated circuits, manufacturers strive to reduce the space requirements of precision resistors.
In addition to the horizontal space requirements, these precision resistors impact vertical space requirements of the associated electronic device. For example, FIG. 1 is a known electronic device 10 having a precision resistor 12 connected to an upper metal layer 14 through a plurality of vias 16 as disclosed in U.S. Pat. No. 7,410,879 to Hill et al. The electronic device includes a first metal layer 18 formed on a substrate 20. The precision resistor is formed on a first dielectric layer 22 that overlies the first metal layer 18 and the substrate 20. Prior to forming the vias 16, a resistor head contact structure 24 is formed over ends 26 of the precision resistor 12. The resistor head contact structure 24 includes a titanium tungsten layer 28 and a second dielectric layer 30.
A thin film resistor layer is generally evaporated or sputtered on the substrate 20 and then patterned and etched to form the resistor 12. In order to operate, the resistor requires an electrical connection to be made to the ends 26, which requires two mask layers, one to shape the resistor 12 and one to form the resistor head contact structures 24. These resistor head contact structures 24 protect the resistor during the via etch that will electrically connect upper metal layer 14 to the resistor 12.
A third dielectric layer 32 is formed overlying the precision resistor 12, the resistor head contact structure 24, and the first dielectric layer 22. The plurality of vias 16 are formed through the third dielectric layer 32 and filled with a conductive material to electrically connect the precision resistor 12 to the upper metal layer 14. Having the precision resistor 12 separated from the first metal layer 18 by the first dielectric layer 22 and having the precision resistor 12 separated from the upper metal layer 14 limits the manufacturer's ability to reduce the size of the electronic device. More particularly, having the first metal layer 18 and the resistor 12 separated by the first dielectric layer 22 adds significant vertical dimensions to the electronic device 10.
FIG. 2 is an isometric view of a known technique for forming precision resistors without vias connecting upper metal layers to the resistors. An electronic device 40 has a tantalum nitride resistor 42 formed directly on exposed portions of an aluminum layer 44 and on a planarized dielectric layer 46 as disclosed in U.S. Pat. No. 5,485,138 to Morris. The aluminum layer 44 is formed on a lower level dielectric layer 48 formed on a gallium-arsenide substrate 50.
The process of forming the resistor 42 includes depositing the aluminum layer 44 directly on the lower level dielectric layer 48 and then patterning and etching the aluminum to form metal lines. The dielectric layer 46 is then formed over the aluminum layer 44. A planarization step smoothes a top surface of the dielectric layer 46. Subsequently, an area of between 1 and 1,000 angstroms of the top of the aluminum layer 44 is exposed. A tantalum nitride layer is then deposited and etched to form the tantalum nitride resistor 42. As can be clearly seen in FIG. 2, the resistor 42 is significantly larger than the aluminum layer 44, which adds additional vertical dimensions to the electronic device 40.
Thin film resistors are attractive for high precision analog and mixed signal applications that have size constraints. Thin-film resistors are generally more precise than diffused and laser trimmed polysilicon resistors. Several parameters define performance of thin film resistors including the resistor's value, the resistor's tolerance, and the temperature coefficient of resistance. The temperature coefficient of resistance provides an adequate means to measure the performance of a resistor. Thin film resistors have superior temperature coefficient of resistance and voltage coefficients of resistance, i.e., a low thermal coefficient of resistance and a low voltage coefficient of resistance. Thin film resistors also have good resistor matching and stability under thermal stress for use in integrated circuits to implement a specific functionality, including biasing of active devices, serving as voltage dividers, and assisting in impedance matching, to name a few.
Many electronic devices utilize high precision thin film resistors, such as operational amplifiers, digital-to-analog converters with high accuracy, implanted medical devices, and radio frequency circuits with high accuracy. Radio frequency (RF) circuits utilize thin film resistors for input/output circuitry in both radio frequency complementary metal-oxide semiconductors (CMOS) and RF silicon germanium technology. In these high precision applications, thin film resistors with a high tolerance, good linearity, a low temperature coefficient of resistance, a high quality factor, and reliability in high current applications are desired. These precision resistors should have a sheet resistance between 100 and 2,000 ohms/square with a temperature coefficient of resistance between −100 and +100 parts per million per degree Celsius. However, integrating thin film resistors into existing product lines can be difficult due to the reduced size of many electronic devices.